I had originally created the `dbmock` package as a separate package
because I thought it provided a nicely readable import path
(dbmock.NewMockDB), and also because it provided a clean separation
between mocks and the code being mocked. However, this meant that
we couldn't use the dbmock package in the database package, which has
been blocking me from getting rid of global mocks entirely.
This just moves the mocks into the database package, so now you'd use it
like database.NewMockDB(), which is maybe actually better IMO because it
corresponds directly to the database.NewDB() function.
This adds some docs for my take on current best practices for testing
with a database dependency. I think the dbmock stores have matured enough that
it's worth sharing the new pattern with others, and while I'm at it, I also added
a section that explains dbtest.NewDB(), with some clarification on when to use
each package.
